1. Field of the Invention
This invention relates to friction-welded parts and a method of friction-welding.
2. Description of Related Art
Conventionally, an iron material is used for cylindrical parts used to constitute a suspension for an automobile, such as an upper link, a lower link, and a radius rod (hereinafter referred to as “suspension part”). However, in order to reduce a vehicle weight, an aluminum alloy material has come to be used more commonly for recent years. For example, Japanese Laid-open Patent Publication No. H11-156562 discloses a suspension part that is formed by friction pressure-welding aluminum-alloy end members onto both sides of an aluminum-alloy pipe-shaped member.
Friction pressure-welding (hereinafter referred to as “friction-welding”) is one of solid-state welding methods and enables joining members of various materials with constant quality, since the friction-welding process does not leave any defect which is often caused when the melt welding process is adopted. Moreover, since pressure-welding does not require special skills such as needed for MIG welding and TIG welding, it is of use in various welding fields.
However, because of friction heat generated during friction welding, it is unavoidable to have a heat affected zone (hereinafter referred to as “HAZ”) at and in the vicinity of a weld interface. The material of HAZ is metallurgically different in properties inclusive of strength from the base material. If a size of HAZ is enlarged beyond a uniform cross section portion to include a part of a non-uniform cross-section portion (whose cross-sectional shape varies), it becomes difficult and complicated to design a suspension part, since stress concentration can easily occur in the non-uniform cross-section portion. Namely, if the size of HAZ reaches the non-uniform cross-section portion, changes in stress due to variations in cross-sectional shape, as well as changes in strength caused by heat effects must be taken into account, which makes it difficult to design a suspension part.
The above-mentioned problem is not limited to a suspension part formed by friction-welding a pipe-shaped member and an end member together. The same problem is also encountered for a general friction-welded part formed by friction-welding a first member and a second member together.
In order to solve the problem with friction welding, the present invention is to provide friction-welded parts to be designed more easily, as well as a method of friction-welding thereof.
A friction-welded part according to the present invention to solve the above-mentioned problem is produced of a first member made of an aluminum alloy material, having a uniform cross-section portion in which the cross-sectional shape does not vary, and a second member made of an aluminum alloy material, having a uniform cross-section portion in which the cross-sectional shape does not vary. The first member and the second member are then friction-welded together to form a friction-welded part, in which the uniform cross-section portions of the first and the second members are joined together by friction welding, having HAZ generated during friction welding formed only in the uniform cross-section portions. It should be noted that the present invention is used to produce a general friction-welded part, as described above. More specifically, the present invention is best used for the production of suspension parts such as a suspension rod for passenger vehicles.
In other words, excellence of the present invention is specifically attributed to forming the uniform cross-section portions of each of the first and the second members to be joined, as well as providing a means for preventing HAZ generated by friction-welding from being created in any other portion than the uniform cross-section portions. According to the present invention, changes in strength caused by heat effects occur only in the uniform cross-section portion and, therefore, design of both first and second members to be joined can be made easily, even if either first or second member includes a portion where a cross-section may vary (i.e. a non-uniform cross-section portion). Namely, while designing a non-uniform cross-section portion of a friction-welded part, the friction-welded part according to the present invention can eliminate the need for taking into consideration any changes in strength caused by heat effects; thus, facilitating design of the part.
It should be noted that, preferably, if the uniform cross-section portion of the first member, and the uniform cross-section portion of the second member to be joined have cylindrical shapes, respectively, both of the uniform cross-section portions should have the same shape and the same size. When the uniform cross-section portions are joined together by friction-welding, and if shapes of the uniform cross-section portions are cylindrical, oxide films already formed on the end surfaces (or the butting face) in the uniform cross-section portions are easily discharged, accompanied by the creation of burrs. In addition, if both of the uniform cross-section portions have an identical shape, friction heat generated at the weld interface is transferred uniformly into the respective uniform cross-section portions, causing a strength distribution to be symmetric with respect to the weld interface. This means that it is much easier to design the welding metal members. Furthermore, the present invention can help to easily produce burrs of bilaterally symmetrical shape made as a result of friction-welding and to determine a desirable condition of friction welding in order to have an effective area of a joint portion friction-welded.
The inventors have discovered that there is no influence on a non-uniform cross-section portion if the following conditions are met. (a) both the uniform cross-section portion of the first member and the uniform cross-section portion of the second member have cylindrical shapes; (b) the uniform cross-section portions of the first and the second members have the same sectional dimension; and (c) the lengths of the uniform cross-section portions are kept 5 mm or longer, respectively, after the friction-welding process. Namely, given a friction-welded part having a uniform cross-section portion whose length is 5 mm or longer after friction-welded, any change in strength which might be caused by heat effects during the friction-welding process is not required to be considered when designing a non-uniform cross-section portion. This makes it much easier to design the non-uniform cross-section portion.
More specifically, it proves that if the uniform cross-section portion is in a cylindrical shape, has a pipe-wall thickness “t” ranging from 2 to 5 mm, and is kept 5 mm long or more after friction-welded, it is guaranteed that HAZ generated by a friction-welding process is restricted within the uniform cross-section portions.
It should be noted that if the lengths of the uniform cross-section portions before friction-welded are (6 mm+t/2) or more, respectively, the lengths of the uniform cross-section portions after friction-welded should be kept 5 mm long or more by shortening each length of the uniform cross-section portion at least by (t/2) through the friction-welding process. When performing friction-welding according to the present invention, HAZ generated by the friction-welding process can be restricted within the uniform cross-section portions. In addition, oxide films already formed on the end surfaces of the uniform cross-section portions are removed from the weld interface. As a result, no variation in joint strength can be observed.
The present invention requires no confinement regarding conformations of the first member and the second member. However, if a friction-welded part of the present invention is applied to a suspension part such as a suspension rod, a pipe-shaped extruded material can be used for the first member and an end member formed of an extrusion material can be used for the second member. Preferably, the uniform cross-section portion of the second member should have a cylindrical shape, and a length thereof should be as long as or shorter than the outer diameter of the uniform cross-section portion, before friction-welded. It should be noted that the uniform cross-section portion of the second member can be formed by cutting and drilling work on a portion of the extrusion material. By setting the length of the uniform cross-section portion of the second member before friction-welded to be equal to or less than the outer diameter of the uniform cross-section portion, it can limit an increase in time and cost required for cutting and drilling work. In addition, if performed likewise, the volume of aluminum alloy removed for cutting and drilling can be minimized, preventing the used material from being unnecessarily wasted. In other words, if the length of the uniform cross-section portion of the second member before friction-welded is greater than the outer diameter of the uniform cross-section portion, it can increase time and cost required for cutting and drilling work. In addition, the volume of aluminum alloy removed for cutting and drilling can also increase, resulting in increase in the wasted material.